1. Field of the Invention
The present invention relates to a self-emissive display device of an active matrix type having a self-emissive element which is driven by a selection driving circuit formed of a thin film transistor (TFT) or the like disposed individually for each pixel, and more particularly to an organic electroluminescence (EL) display device of the active matrix type.
2. Description of the Related Art
Because self-emissive display devices are smaller and consume less electric power than CRTs, and exhibit no dependency on the viewing angle unlike LCDs, EL display devices employing EL elements are attracting attentions in recent years as potential replacements for CRTs and LCDs. Further, EL display devices provided with TFTs as switching elements for driving such EL elements, for example, are now being developed.
FIG. 1 is an equivalent circuit diagram of an organic EL display device. In the device, a plurality of gate lines 1 extending in a row direction are disposed, and a plurality of data lines 2 and a plurality of driving lines 3 are disposed in a column direction crossing the gate lines. The driving line 3 is connected to a power source PV, which supplies a positive constant voltage of, for example, 10 V using a ground voltage as a reference voltage. A selection TFT 4 is connected to each intersection between the gate line 1 and the data line 2. The selection TFT 4 is of a double gate configuration in which two TFTs 4a and 4b are connected in series. The TFTs 4a and 4b of the selection TFT 4 each have a gate connected to the gate line 1, and a drain of the TFT 4a is connected to the data line 2. The selection TFT 4b has a source connected to one electrode of a storage capacitor 5 and a gate of a driving TFT 6, which has a drain connected to the driving line 3 and a source connected to an anode of an organic EL emissive element 7. The cathode of the organic EL emissive element 7 is connected to ground. A capacitor line 9 extending in the column direction is connected to the other electrode of the storage capacitor 5.
The gate lines 1 are connected to an unillustrated gate line driver, and sequentially receive a gate signal applied by the gate line driver. The gate signal is a binary signal which assumes either an ON or OFF state. The signal has a positive predetermined voltage in the ON state, and 0V in the OFF state. The gate line driver turns on the gate signal on the predetermined gate line selected among the plurality of gate lines 1 connected thereto. When the gate signal is turned on, TFTs of all the selection transistors 4 connected to that gate line 1 are turned on, and the data line 2 and the gate of a driving transistor 6 are connected through the selection transistor 4. To the data line 2, a data signal determined in accordance with a displayed image is applied from a data line driver 8, and is therefore applied to the gate of the driving transistor 6 and charged in the storage capacitor 5. The driving transistor 6 connects the driving line 3 and the organic EL emissive element 7 at an electrical conductivity corresponding to the value of the data signal. As a result, a current corresponding to the data signal is supplied from the driving line 3 to the organic EL emissive element 7 through the driving transistor 6, whereby the organic EL emissive element 7 emits light at a luminance corresponding to the data signal. The storage capacitor 5 forms a static capacitor with another electrode, such as a dedicated capacitor line 9 or the driving line 3, and is capable of storing the data signal for a predetermined period of time. Even after a particular gate line 1 is deselected to turn off the selection transistor 4 when the gate line driver selects another gate line 1, the data signal is stored by the storage capacitor 5 for one vertical scanning period, during which the driving transistor 6 maintains the above electrical conductivity. The organic EL emissive element can thereby continue to emit light at the same luminance.
For the active matrix organic EL display device operating in the above-described principles, it should be noted that the term xe2x80x9cselection driving circuitxe2x80x9d in this specification refers generally to a circuit having the selection transistor 4 and the driving transistor 6 described above and applying a signal simultaneously selecting one or more display elements, such as the gate signal, and the data signal determined by the displayed image to thereby supply a current corresponding to the data signal to a predetermined display element.
FIG. 2 is a cross sectional view of an organic EL display device of the active matrix type. A plurality of driving TFTs 6 are disposed on a glass substrate 11. In the driving TFT 6, a gate electrode 6G is provided facing a source 6S, a channel 6C, and a drain 6D with an interlayer insulating film 12 interposed between them. In the illustrated example, the driving TFT 6 is of a bottom gate type with the gate electrode 6G disposed below the channel 6C. An interlayer insulating film 13 is formed on the driving TFT 6, and the data line 2 and the driving line 3 are disposed on top of the film 13. The driving line 3 is connected to the drain 6D of the driving TFT 6 through a contact hole. On such elements a planarization insulating film 14 is formed, on which the organic EL emissive element 7 is disposed for each pixel. The organic EL emissive element 7 is composed of an anode 15 formed of a transparent electrode of, for example, ITO (indium tin oxide), a hole transportation layer 16, an emissive layer 17, an electron transportation layer 18, and a cathode 19 of a metal, such as aluminum, stacked in the above order. Holes injected from the anode 15 to the hole transportation layer 16 and electrons injected from the cathode 19 to the electron transportation layer 18 are recombined in the emissive layer 17 to cause emission of light, which is transmitted through the transparent electrode 15 and the glass substrate 11 to outside, as indicated by the arrow in the figure. The anode 15 and the emissive layer 17 are individually formed for each pixel, while the hole transportation layer 16, the electron transportation layer 18, and the cathode 19 are shared by respective pixels.
FIG. 3 shows a correlation curve between a voltage VEL applied to the organic EL emissive element 7 and the luminance of light emitted at the corresponding moment. Light is not emitted regardless of the voltage value when the voltage VEL is at a predetermined value V0 or smaller. After light emission begins when the voltage VEL exceeds the predetermined value V0, the luminance is increased with an increase in the voltage VEL. When an organic EL element is used as an emissive element in a display device, images are displayed by controlling the voltage VEL applied to the EL emissive element to fall within a range between a minimum voltage Vmin for a current observed when the EL emissive element emits dim light at a predetermined luminance Lmin and a maximum voltage VMAX for a current corresponding to a maximum luminance LMAX defined as the luminance resulting in a predetermined contrast ratio to the luminance Lmin of, for example, 100:1. Although it is possible to cause stronger light emission and achieve a higher contrast by setting the voltage VEL to an even higher value, the life of an organic EL element is shortened by strong light emission, which requires a larger amount of current. Therefore, in view of both the life and current consumption, the maximum luminance and contrast are set at a level appropriate to the circumstances under which the display device is employed.
FIG. 4 is a circuit diagram illustrating only the power source PV, the driving TFT 6, and the EL emissive element 7 for one pixel, extracted from the circuit diagram of FIG. 1. As can be seen from the figure, the driving TFT 6 and the organic EL emissive element 7 are connected in series between the power source PV and ground. A current IEL flowing through the organic EL emissive element 7 is supplied from the power source PV to the organic EL emissive element 7 via the driving transistor 6, and can be controlled by changing a gate voltage VG of the driving transistor 6. As described above, the data signal is supplied to the gate electrode, and the gate voltage VG assumes a value corresponding to the data signal. FIGS. 5A and 5B show a correlation between the gate voltage VG of the driving transistor 6 and the luminance of light emitted from the EL emissive element 7 when the driving transistor 6 is a p-type TFT and an n-type TFT, respectively. When the driving transistor 6 is a p-type TFT, the maximum luminance LMAX is achieved with the gate voltage VG of 4.5 V. The luminance decreases with an increase in the gate voltage VG, and drops to the minimum luminance Lmin with the gate voltage VG of 6.5 V. Further, light emission cannot be observed when the gate voltage VG reaches approximately 8 V. On the other hand, when the driving transistor 6 is an n-type TFT, light emission begins with application of a gate voltage VG of about 3 V, and the luminance increases with an increase in the gate voltage VG. The minimum luminance Lmin and the maximum luminance LMAX are obtained when the gate voltage VG is 4.5V and 6.5 V, respectively.
From the foregoing, it can be concluded that the gate voltage of the driving transistor 6, i.e. the data signal, should be output after being adjusted to a value ranging from VG min of 4.5 to VGMAX of 6.5 V and corresponding to the displayed tone in order to control the luminance of light emitted from the organic EL emissive element 7. FIG. 6 shows an example of chronological change of the data signal to be applied to the gate of the driving transistor 6.
However, applying such a data signal requires a power source capable of supplying a large amount of current to deal with the maximum data signal. The data signal is produced by an unillustrated external data signal processing circuit for processing an externally applied video signal into a signal optimum for use as the gate voltage applied to the driving transistor 6. When the voltage of the data signal to be supplied from the processing circuit is high, the driving voltage of such a data signal processing circuit must also be raised, leading to an increase in power consumption.
Especially when the voltage VGMAX exceeds 5 V, the data signal processing circuit must be designed to have a high withstanding voltage, making it difficult to manufacture the circuit through a common CMOS process. Accordingly, the above-described self-emissive display device of the active matrix type consumes a large amount of electric power, and is expensive to manufacture.
An object of the present invention is to provide an active matrix type self-emissive display device which consumes less electric power during operation, and which can be inexpensively manufactured.
To achieve the above object, the present invention comprises the following characteristics.
A self-emissive display device of an active matrix type according to the present invention comprises a plurality of selection lines extending in a row direction, a plurality of data lines extending in a column direction, a selection driving circuit disposed corresponding to and a vicinity of each intersection between said selection line and said data line, and an emissive element connected to each selection driving circuit, wherein said emissive element emits light at a luminance in accordance with a data signal in response to a current supplied to said emissive element in accordance with the data signal supplied to said data line when said selection driving circuit is turned on in response to a selection signal supplied to said selection line, and said emissive element is connected between a first power source for applying a voltage of a first polarity with respect to a reference voltage and a second power source for applying a voltage of a polarity opposite to said first polarity.
According to one aspect of the present invention, a display device of an active matrix type is provided wherein each of a plurality of pixels is provided with a selection transistor, an element driving transistor, and an emissive element, said emissive element emits light in response to a current supplied to said emissive element through said element driving transistor in accordance with a data signal supplied to said element driving transistor through said selection transistor when the selection transistor is turned on in response to a selection signal supplied thereto, and said element driving transistor and said emissive element are connected in series between a first power source for applying a voltage of a first polarity with respect to a reference voltage and a second power source for applying a voltage of a polarity opposite to said first polarity.
According to other aspect of the present invention, a self-emissive display device of an active matrix type comprises a plurality of selection lines extending in a row direction and sequentially receiving a selection signal, a plurality of data lines extending in a column direction and receiving a data signal in accordance with displayed content, and a selection transistor, an element driving transistor, and an emissive element, provided in the vicinity of each intersection between said selection line and said data line, wherein said selection transistor has a gate connected to a corresponding one of said plurality of selection lines, said element driving transistor has a gate connected to a corresponding one of said plurality of data lines through said selection transistor, said emissive element has an anode connected to said element driving transistor and receiving a current from a first power source through said element driving transistor, and said emissive element has a cathode connected to a second power source for shifting a data voltage.
According to other aspect of the present invention, said first power source is a positive power source, and said second power source is a negative power source.
According to other aspect of the present invention, said selection signal has a first potential of a predetermined polarity when selected, and a second potential of a polarity opposite to that of said first potential when unselected.
According to other aspect of the present invention, said selection signal has a first potential of a predetermined polarity when selected, and, when unselected, a second potential of a polarity opposite to that of said first potential to turn off said selection driving circuit or said selection transistor, said second potential being larger than the absolute value of a minimum required luminance voltage of said data signal or the absolute value of a maximum required luminance voltage thereof.
According to other aspect of the present invention, said emissive element is an organic electroluminescence element.
According to the present invention, a self-emissive element is connected between a first power source for supplying a voltage of a first polarity and a second power source for supplying a voltage of a polarity opposite to the first polarity, so that the data signal can be shifted by an amount corresponding to the second power source voltage to contribute to reduction in voltage, whereby power consumption of the display device can be reduced.
Further, the data signal processing circuit does not require a transistor with a high withstanding voltage, and therefore can be manufactured through a common CMOS process, thereby reducing the cost to manufacture the display device.